Production of aluminum by the Hall-Heroult process makes use of a cell having a chamber containing alumina dissolved in a molten cryolite electrolyte bath. It is standard practice to add aluminum fluoride regularly to the cryolite so that the NaF/AlF3 mass ratio is maintained at about 0.70-1.20.
The cryolite bath is covered by a solid crust that is punctured regularly when molten aluminum is tapped from the cell. Increasing the area of the open crust holes results in more fluoride evolution from the smelting pot, thereby increasing load on the pot scrubber and the resulting smelter fluoride emission level.
Increasing the average area of open crust holes in a pot line also increases variations in the bath ratio, resulting in poorer cell performance. This occurs because the amount of fluoride evolved from individual pots fluctuates while each pot continues to receive a relatively constant supply of fluoride in reacted ore from the dry scrubber line, plus the same daily maintenance supply of aluminum fluoride. These factors make it desirable to quantify the effects of pot operating practices on fluoride evolution in order to prioritize various efforts to minimize fluoride evolution.
In the prior art some attempts have been made to control aluminum fluoride additions to smelting cells. However, the prior art attempts each suffer from one or more serious disadvantages making them less than entirely suitable for their intended purpose.
Desclaux et al. U.S. Pat. No. 4,668,350 issued May 26, 1987 represents an effort in the prior art to control the rate of addition of aluminum fluoride to a cryolite-based electrolyte in an aluminum production cell. The claimed method requires regular measurements of cell temperature, either directly or by means such as a thermocouple inserted in the side wall, or the floor, or in a cathode current collector in the cell floor.
A principal objective of the present invention is to provide a process for controlling additions of aluminum fluoride to individual aluminum electrolysis cells, thereby to improve energy efficiency.
A related objective of the invention is to provide a process for controlling inspections and repairs for crust holes in aluminum production cells so that such inspections and repairs are performed where and as needed.
A further objective of our invention is to reduce fluoride emissions from aluminum production cells having cryolite-based molten electrolytes.
Additional objectives and advantages of our invention will become apparent to persons skilled in the art from the following detailed description of some particularly preferred embodiments.